Pressure sensor having multiple pressure cells and sensitivity estimation methodology

ABSTRACT

A pressure sensor ( 20 ) includes a test cell ( 32 ) and sense cell ( 34 ). The sense cell ( 34 ) includes an electrode ( 42 ) formed on a substrate ( 30 ) and a sense diaphragm ( 68 ) spaced apart from the electrode ( 42 ) to produce a sense cavity ( 64 ). The test cell ( 32 ) includes an electrode ( 40 ) formed on the substrate ( 30 ) and a test diaphragm ( 70 ) spaced apart from the electrode ( 40 ) to produce a test cavity ( 66 ). Both of the cells ( 32, 34 ) are sensitive to pressure ( 36 ). However, a critical dimension ( 76 ) of the sense diaphragm ( 68 ) is less than a critical dimension ( 80 ) of the test diaphragm ( 70 ) so that the test cell ( 32 ) has greater sensitivity ( 142 ) to pressure ( 36 ) than the sense cell ( 34 ). Parameters ( 100 ) measured at the test cell ( 32 ) are utilized to estimate a sensitivity ( 138 ) of the sense cell ( 34 ).

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to pressure sensors. Morespecifically, the present invention relates to a pressure sensor havingmultiple pressure cells of differing sensitivities and methodology formeasuring sensitivity of the pressure sensor.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (MEMS) devices are semiconductor deviceswith embedded mechanical components. MEMS devices include, for example,pressure sensors, accelerometers, gyroscopes, microphones, digitalmirror displaces, micro fluidic devices, and so forth. MEMS devices areused in a variety of products such as automobile airbag systems, controlapplications in automobiles, navigation, display systems, inkjetcartridges, and so forth. Capacitive-sensing MEMS devices designs arehighly desirable for operation in miniaturized devices due to their lowtemperature sensitivity, small size, and suitability for low cost massproduction.

A microelectromechanical systems (MEMS) pressure sensor typically uses apressure cavity and a membrane element, referred to as a diaphragm, thatdeflects under pressure. In some configurations, a change in thedistance between two plates, where one of the two plates is the movablediaphragm, creates a variable capacitor to detect strain (or deflection)due to the applied pressure over the area. Process variation on criticaldesign parameters, such as the width of a MEMS pressure sensordiaphragm, can affect the sensitivity of a pressure sensor. For example,a small difference in the width of a MEMS pressure sensor diaphragm canresult in a large difference in sensitivity, relative to thepredetermined nominal, or design, sensitivity for the pressure sensor.Accordingly, the sensitivity of each MEMS pressure sensor is typicallycalibrated individually. The equipment used for this calibration can becostly and difficult to maintain. Additionally, calibration can be slowdue to the imposition of a physical pressure stimulus on the pressuresensor in order to calibrate the pressure sensor. Individual calibrationof MEMS pressure sensors by imposing a physical pressure stimulusundesirably increases costs associated with the pressure sensor and/orcan introduce error in pressure measurements.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description and claims when considered inconnection with the Figures, wherein like reference numbers refer tosimilar items throughout the Figures. Elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale.

FIG. 1 shows a simplified top view of a pressure sensor in accordancewith an embodiment;

FIG. 2 shows a side sectional view of the pressure sensor along sectionlines 2-2 of FIG. 1;

FIG. 3 shows a simplified block diagram of a pressure sensing system;

FIG. 4 shows a flowchart of a pressure sensor sensitivity estimationprocess in accordance with another embodiment; and

FIG. 5 shows a table of equations for deriving the sensitivity of asense cell of the pressure sensor from the sensitivity of a test cell.

DETAILED DESCRIPTION

Embodiments of the present invention entail a pressure sensor andmethodology for estimating the sensitivity of the MEMS pressure sensor.The pressure sensor includes multiple pressure sensor cells on a singledie having different sensitivities. Sense signals from one set of thepressure sensor cells (i.e., test cells) may be utilized to estimate thesensitivity of another set of the pressure sensor cells (i.e., sensecells). These different sensitivities can be achieved by fabricating thetest cells with a greater diaphragm width than the sense cells. Thewidths of the test and sense diaphragms can vary slightly from designspecifications due to process variation. Thus, the widths of the testand sense diaphragms are only approximately known for the test and sensecells. However, the difference between the two widths is well knownregardless of process variation. Knowledge of the difference in widthsof the test and sense diaphragms is utilized herein to provide anestimate of the sensitivity of the sense cells relative to the testcells in order to determine the sensitivity of the pressure sensor. Sucha pressure sensor and methodology can reduce test costs, provideimproved feedback for process control, and enable sensitivity estimationwithout imposing a physical stimulus calibration signal.

Referring now to FIGS. 1 and 2, FIG. 1 shows a simplified top view of aMEMS pressure sensor 20 in accordance with an embodiment, and FIG. 2shows a side sectional view of pressure sensor 20 along section lines2-2 of FIG. 1. Pressure sensor 20 generally includes a sense structure22 and a reference structure 24. Sense structure 22 and referencestructure 24 may be fabricated on an insulating layer 26, such as anitride layer, formed on a surface 28 of a substrate 30. Insulatinglayer 26 can comprise any suitable insulative or dielectric materiallayer selected according to the requirements of a given pressure sensorimplementation.

Sense structure 22 includes sense cells 32, 34 that are configured in aninterleaved arrangement, i.e., an alternating arrangement of sense cells32 with sense cells 34. In general, individual sense cells 32, 34 ofsense structure 22 are sensitive to ambient pressure 36, represented byan arrow and labeled P in FIG. 2. Reference structure 24 includesreference cells 38. Unlike sense cells 32, 34, reference cells 38 ofreference structure 24 are largely insensitive to ambient pressure 36.In alternative embodiments, sense cells 32, 34 need not be interleaved,but may instead be arranged in other structural configurations.

Both sets of sense cells 32 and sense cells 34 are sufficientlysensitive to detect ambient pressure 36. However, as will be discussedin significantly greater detail in connection with FIGS. 3 and 4, sensecells 32 are implemented within pressure sensor 20 to estimate thesensitivity of sense cells 34. Once their sensitivity is estimated,sense cells 34 are used within pressure sensor 20 to detect andsubsequently output a measure indicative of pressure 36. Thus, in orderto distinguish them, sense cells 32 will be referred to hereinafter astest cells 32 and sense cells 34 will continue being referred to assense cells 34.

Sense structure 22 includes electrodes 40, 42, 44, 46, 48, and 50 formedin or on insulating layer 26. Likewise, electrodes 52, 54, and 56 ofreference structure 24 may be formed in or on insulating layer 26. InFIG. 1, electrodes 40, 42, 44, 46, 48, and 50 of sense structure 22 areillustrated in phantom using dotted lines, due to their location beneatha common electrode 58. Likewise, electrodes 52, 54, and 56 of referencestructure 24 are also illustrated in phantom using dotted lines, due totheir location beneath a common electrode 60 and a cap layer 62. FIGS. 1and 2 are illustrated using various shading and/or hatching todistinguish the different elements produced within the structural layersof the devices, as will be discussed below. These different elementswithin the structural layers may be produced utilizing current andupcoming surface micromachining techniques of depositing, patterning,etching, and so forth. Accordingly, although different shading and/orhatching may be utilized in the illustrations, the different elementswithin the structural layers can be formed out of the same material,such as polysilicon, single crystal silicon, and the like.

Electrodes 42, 46, and 50 represent a set of sense capacitor bottomplate electrodes for sense cells 34, while electrodes 40, 44, and 48represent another set of sense capacitor bottom plate electrodes fortest cells 32. Since test cells 32 are interleaved with sense cells 34,electrodes 40, 44, and 48 are correspondingly configured in aninterleaved arrangement with electrodes 42, 46, and 50. In someembodiments, a geometry of individual electrodes of the first set ofelectrodes 42, 46, and 50 may match a geometry (i.e., width, length, andthickness) of individual electrodes of the second set of electrodes 40,44, and 48, i.e., the geometries are substantially similar. However,matching geometries is not a limitation.

Referring still to sense structure 22, common electrode 58 represents acapacitor top plate electrode for sense cells 34 and test cells 32respectively. Common electrode 58 is overlying, spaced apart from, andconfigured in connection with electrodes 40, 42, 44, 46, 48, and 50, toproduce sense cavities 64 for sense cells 34 and test cavities 66 fortest cells 32. Cavities 64 and 66 are represented as separate cavitiesherein. However, in alternative embodiments, cavities 64 and 66 may beformed as a common cavity. Cavities 64 and 66 may be vacuum chambers orchambers filled with a suitable gas at a given controlled pressure.

Common electrode 58 anchors to the surface of insulating layer 26 forestablishing portions of common electrode 58 corresponding to sensediaphragms 68 for sense cells 34 and for establishing other portions ofcommon electrode 58 corresponding to test diaphragms 70 for test cells32. For example, common electrode 58 anchors to the insulating layer 26about a perimeter 72 of common electrode 58 and at desired anchorlocations internal to the perimeter, such as indicated by referencenumerals 74, to establish cavities 64 and 66 and to distinguish sensediaphragms 68 from test diaphragms 70.

In general, an area of each of sense diaphragms 68 is less than an areaof each of test diaphragms 70. More particularly, each of sensediaphragms 68 is characterized by a width 76 and a length 78. Likewise,each of test diaphragms 70 is characterized by a width 80 and a length82. In an embodiment, length 78 of each of sense diaphragms 68 equalslength 82 of each of test diaphragms 70. However, width 80 of each oftest diaphragms 70 is greater than width 76 of each of sense diaphragms68. Since lengths 78 and 82 are equal, and width 80 of test diaphragms70 is greater than width 76 of sense diaphragms 68, it follows that anarea of each of test diaphragms 70 is greater than an area of each ofsense diaphragms 68.

The greater width 80 of each of test diaphragms 70 causes testdiaphragms 70 to deflect more than sense diaphragms 68 in response topressure 36, thus resulting in a greater sensitivity of test cells 32 topressure 36 than sense cells 34. Accordingly, in the illustratedembodiment, widths 76 and 80 are critical dimensions that directlyaffect the sensitivity of sense cells 22 and test cells 24,respectively. In some embodiments, width 80 may be approximately ten totwenty percent greater than width 76 so that test cells 32 areapproximately twice as sensitive to pressure 36 as sense cells 34. Thisgreater sensitivity is exploited when estimating the sensitivity ofsense cells 34, as will be discussed in connection with FIGS. 3 and 4.

MEMS pressure sensor 20 further includes a conductive runner 84electrically coupled to electrodes 42, 46, and 50 to provide electricalaccess external to sense cells 34 of sense structure 22. Anotherconductive runner 86 is electrically coupled to electrodes 40, 44, and48 to provide electrical access external to test cells 32. Additionallya conductive runner 88 is electrically coupled to common electrode 58.

Referring now to reference structure 24 presented in FIGS. 1 and 2,electrodes 52, 54, and 56 represent a set of sense capacitor bottomplate electrodes for reference cells 38. Common electrode 60 isoverlying, spaced apart from, and configured in connection withelectrodes 52, 54, and 56 to produce reference cavities 90 for referencecells 38. Cavities 90 are represented as separate cavities herein.However, in alternative embodiments, cavities 90 may be formed as acommon cavity. Common electrode 60 anchors to the surface of insulatinglayer 26 for establishing reference diaphragms 92 for reference cells38. For example, in FIG. 1, common electrode 60 anchors to insulatinglayer 26 about a perimeter of common electrode 60 and at desired anchorlocations internal to the perimeter, to establish cavities 90 and todistinguish reference diaphragms 92 from one another.

Cap layer 62 is formed in contact with diaphragms 92. Cap layer 62 maybe a relatively thick layer of, for example, tetraethyl orthosilicate(TEOS), which makes diaphragms 92 largely insensitive to pressure. Assuch, diaphragms 92 may be referred to hereinafter as referenceelectrodes 92. A conductive runner 94 is electrically coupled toelectrodes 52, 54, and 56 of reference structure 34 to provideelectrical access external to sense cells 34 of sense structure 22.Another conductive runner 96 is electrically coupled to common electrode60. It should be observed in FIG. 1 that common electrode 60 isillustrated in phantom using dashed lines, due to its location beneathcap layer 62.

In general, sense cells 34 form a capacitor between diaphragms 68 andelectrodes 42, 46, and 50. That is, a sense signal, referred to hereinas a sense capacitance 98, labeled C_(S1), is produced between sensediaphragms 68 and electrodes 42, 46, and 50 (i.e., the differencebetween C_(S1) ⁺ and C_(S1) ⁻) that varies in response to pressure 36.Likewise, test cells 34 form a capacitor between diaphragms 70 andelectrodes 40, 44, and 48. That is, a test signal, referred to herein asa test capacitance 100, labeled C_(S2), is produced between testdiaphragms 70 and electrodes 40, 44, and 48 (i.e., the differencebetween C_(S2) ⁺ and C_(S2) ⁻) that also varies in response to pressure36. A distinction of MEMS pressure sensor 20 is that the sensitivity oftest cells 32 producing capacitance 100 is different from thesensitivity of sense cells 34 producing sense capacitance 98. As such,test capacitance 100 may be greater than sense capacitance 98 inresponse to pressure 36 because width 80 of diaphragm 70 of each testcell 32 is greater than width 76 of diaphragm 68 of each sense cell 34.

Reference cells 38 also form a capacitor between each of electrodes 92and reference electrodes 52, 54, and 56. Thus, a reference capacitancesignal 102, C_(R), is formed between electrodes 92 and referenceelectrodes 52, 54, and 56 (i.e., the difference between C_(R) ⁺ andC_(R) ⁻). However, reference capacitance signal 102 does not vary inresponse to pressure 36 due to the presence of cap layer 62. In anembodiment, conductive runner 88 for sense structure 22 and conductiverunner 96 for reference structure 24 are interconnected to form a commonnode 104 between sense structure 22 and reference structure 44.

A control circuit 106 is configured to measure the ratio of sensecapacitance signal 98 to reference capacitance signal 102 (i.e.,C_(S1)/C_(R)). Higher pressure 36 increases sense capacitance 98,C_(S1), but has little effect on reference capacitance 102, C_(R).Therefore the ratio of sense capacitance 98 to reference capacitance 102(i.e., C_(S1)/C_(R)) increases as pressure 36 increases. This value canbe converted into an output signal 108, i.e., a measure indicative ofpressure 36.

In the views of pressure sensor 20 shown in FIGS. 1 and 2, sensestructure 22 is illustrated as having three test cells 32 and threesense cells 34. Likewise, reference structure 24 is illustrated ashaving three reference cells 38. However, it should be understood bythose skilled in the art that pressure sensor 20 may have any suitablequantity of test, sense, and reference cells 32, 34, 38, respectively,and their associated diaphragms/electrodes. Additionally, pressuresensor 20 may include other features on substrate 30 such as shieldlines, a guard ring, and so forth that are not included in FIGS. 1 and 2for simplicity of illustration.

Pressure sensor 20 is illustrated with generally rectangular diaphragmshaving a width that is less than a length of the rectangular diaphragms.However, the diaphragms need not be rectangular, but may instead beother shapes (e.g., squares, circles, multi-sided elements, and soforth) with test cells 32 having greater sensitivity than sense cells 34in order to provide sensitivity estimation.

FIG. 3 shows a shows a simplified block diagram of a device 109. Device109 includes pressure sensor 20, control circuit 106, and any otherapplication specific integrated circuit (ASIC) 111 or ASICs 111appropriate for the operation of device 109. Device 109 may be apressure sensing system for an automotive application such as for airbagpressure sensing, oil pressure sensing, HVAC pressure sensing, and othervarious automotive pressure sensing applications. Alternatively, device109 may be a global positioning system (GPS) unit, smartphone, tablet,sports watch, weather station, or any other industrial application inwhich pressure sensing may be utilized. Regardless of the particulardevice 109, the sensitivity of pressure sensor 20 included in device 109can be estimated prior to or following its installation within device109 without imposition of a physical stimulus calibration signal, anddevice 109 can be calibrated as needed.

FIG. 4 shows a flowchart of a pressure sensor sensitivity estimationprocess 110 in accordance with another embodiment. Pressure sensorsensitivity estimation process 110 is performed to estimate thesensitivity of sense cells 34 (FIG. 1) of pressure sensor 20 (FIG. 1)using the higher sensitivity test cells 32 (FIG. 1). Estimation process110 can be performed under ambient pressure conditions, e.g., standardatmospheric pressure, without imposing a physical pressure calibrationstimulus in excess of atmospheric pressure.

Estimation process 110 begins with a task 112. At task 112, ambientpressure 36 (FIG. 1) is measured in the location at which pressuresensor 20 is being tested. Pressure 36 may be measured using anysuitable and highly accurate pressure measurement device.

Process 110 continues with a task 114. At task 114, sense capacitance 98(FIG. 1) is determined for sense cells 34 (FIG. 1).

A task 116 is performed in conjunction with task 114. At task 116, testcapacitance 100 is determined for test cells 32 (FIG. 1)

Sensitivity estimation process 110 continues with a task 118. At task118, the sensitivity of pressure sensor 20 (FIG. 1), and in particular,sense cells 34 (FIG. 1) is estimated using sense capacitance 98 and testcapacitance 100. The details of estimation task 118 are discussed inconnection with FIG. 5.

Following task 118, a task 119 may be performed. At task 119, theresults obtained from estimation task 118 may be utilized to calibrateor otherwise trim pressure sensor 20 in accordance with knownmethodologies. Accordingly, following task 119, pressure sensorsensitivity estimation process 110 ends.

FIG. 5 shows a set 120 of equations for deriving the sensitivity ofpressure sensor 20, and particularly, for estimating the sensitivity ofsense cells 34 (FIG. 1) of sense structure 22 (FIG. 1) utilizing thehigher sensitivity test cells 32. Set 120 reveals that a relationshipcan be established between sensitivity and the geometry of a pressuresensing cell, e.g., sense cell 34 and test cell 32. Three parametershave a strong effect on the sensitivity of a pressure sensing cell. Asshown in the provided figure of an exemplary pressure sensing cell 122,these parameters include a width 124, W, of a diaphragm 126 of pressurecell 122, a depth 128, D, of a cavity 130 underlying diaphragm 126, anda thickness 132, T, of diaphragm 126. Accordingly, a diaphragmsensitivity 133, SENS, can be expressed as a function of width 124,depth 128, and thickness 132 as represented by a generalized functionalequation 134.

Therefore, an equation 136 represents a value of a sensitivity 138,SENS₁, of sense cells 34 (FIG. 1) of sense structure 22 (FIG. 1) havingwidth 76 of sense diaphragms 68 (FIG. 2), depth 128 of cavities 64 (FIG.2), and thickness 132 of diaphragms 68. Likewise, an equation 140represents a value of a sensitivity 142, SENS₂, of test cells 32(FIG. 1) of sense structure 22 having width 80 of test diaphragms 70(FIG. 2), depth 128 of cavities 66 (FIG. 2), and thickness 132 ofdiaphragms 70, where width 80 is greater than width 76 by a valuerepresented by ω.

Given the relationship between sensitivity and the geometry of apressure sensing cell, set 120 further reveals that the value ofsensitivity 138 of sense cells 34 (FIG. 1) can be derived relative tothe value of sensitivity 142 of test cells 32 (FIG. 1). Rearranging theterms of equation 140 yields an equation 144, and substituting theappropriate terms of equation 144 into equation 138 yields an equation146 in which sensitivity 138 of sense cells 34 is a function ofsensitivity 142 of test cells 32, as well as, diaphragm widths 76 and80. Rearranging the terms of equation 146 yields a sensitivity equation148 for sense cells 34 in which sensitivity 138 is a function ofsensitivity 142 of test cells 32 and a ratio of diaphragm width 76 todiaphragm width 80. Thus, sensitivity 138 is related to sensitivity 142by two parameters. These parameters include width 76 of sense diaphragms68 (FIG. 2) of sense cells 34 (FIG. 2) and the difference, ω, betweendiaphragm width 80 and diaphragm width 76. Width 76 is knownapproximately, but not exactly since width 76 may vary from its designwidth due to some process variations, e.g., over or under etch. Thisdifference, ω, is well known since this is the difference betweendiaphragm widths 80 and 76, regardless of process variation resulting insome over or under etch.

As further shown in set 120, sense capacitance 98 can be defined as afunction of sensitivity 138, ambient pressure 36, and zero pressureoffset, ZPO₁, represented by a capacitance equation 150. Likewise, testcapacitance 100 can be defined as a function of sensitivity 142, ambientpressure 36, and zero pressure offset, ZPO₂, as represented by acapacitance equation 152. Zero pressure offset is the theoretical outputof pressure sensor 20 at zero pressure. Due to their structuralconfiguration, it can be assumed that the zero pressure offset, ZPO₂,for test cells 32 is equal to the zero pressure offset, ZPO₁, for sensecells 34.

Accordingly, with ZPO₂=ZPO₁, capacitance equations 150 and 152 can becombined and rearranged to derive a sensitivity equation 154 for testcells 34, where sensitivity 142 is shown to be a function of sensecapacitance 98, test capacitance 100, sensitivity 138 of sense cells 34,and pressure 36.

Sensitivity equation 154 for test cells 32 can be combined withsensitivity equation 148 for sense cells 34 to yield another equation156. Equation 156 can be mathematically rearranged as represented by asensitivity equation 158 in order to derive sensitivity 138 of sensecells 34. Accordingly, sensitivity 138 can be shown to be a function ofwidth 76 (approximately known), the difference, ω, between width 80 andwidth 76 (exactly known), sense capacitance 98 at pressure 36(measured), test capacitance 100 at pressure 36 (measured), and pressure36 (measured). Thus, through the execution of pressure sensorsensitivity estimation process 110, the estimated sensitivity 138 ofsense cells 34 of pressure sensor 20 can be determined utilizingparameters derived from the higher sensitivity test cells 32.

Exemplary equation 158 is provided herein for illustrative purposes. Inpractice, however, there may be deviations from the ideal that may callfor the inclusion of scaling constants and/or other terms, not shown forsimplicity of illustration. Some additional terms may be added tocompensate for higher order effects that are not in the theoreticalmodels.

It is to be understood that certain ones of the process blocks depictedin FIG. 4 may be performed in parallel with each other or withperforming other processes. In addition, it is to be understood that theparticular ordering of the process blocks depicted in FIG. 4 may bemodified, while achieving substantially the same result. Accordingly,such modifications are intended to be included within the scope of theinventive subject matter. In addition, although particular systemconfigurations are described in conjunction with FIGS. 1-2, above,embodiments may be implemented in systems having other architectures, aswell. These and other variations are intended to be included within thescope of the inventive subject matter.

An embodiment of a pressure sensor comprises a sense cell having a firstelectrode formed on a substrate and a sense diaphragm overlying andspaced apart from the first electrode to produce a sense cavity. Thepressure sensor further comprises a test cell having a second electrodeformed on the substrate and a test diaphragm overlying and spaced apartfrom the second electrode to produce a test cavity. Each of the sensecell and the test cell are sensitive to pressure, and a first area ofthe sense diaphragm is less than a second area of the test diaphragm.

An embodiment of a method of determining a sensitivity of a pressuresensor comprises measuring an ambient pressure, determining a firstsense signal between a first electrode and a sense diaphragm of a sensecell of the pressure sensor at the ambient pressure, and determining asecond sense signal between a second electrode and a test diaphragm of atest cell at the ambient pressure. The sensitivity of the sense cell isestimated using the measured ambient pressure, and the first and secondsense signals.

The embodiments of a MEMS pressure sensor and a method of estimating thesensitivity of the MEMS pressure sensor. The pressure sensor includesmultiple pressure sensor structures having different sensitivitiesformed on a single die. Atmospheric pressure (approximately 100 kPa) issufficient to deflect each diaphragm differently. Each pressure sensorcan thus have a different sense signal at atmospheric pressure. Thesense signals from a higher sensitivity set of the pressure sensorstructures may be utilized to estimate the sensitivity of another set ofthe pressure sensor structures. Such a pressure sensor and methodologycan reduce test costs, provide improved feedback for process control,and enable sensitivity estimation without imposing a physical stimuluscalibration signal.

While the principles of the inventive subject matter have been describedabove in connection with specific apparatus and methods, it is to beclearly understood that this description is made only by way of exampleand not as a limitation on the scope of the inventive subject matter.The various functions or processing blocks discussed herein andillustrated in the Figures may be implemented in hardware, firmware,software or any combination thereof. Further, the phraseology orterminology employed herein is for the purpose of description and not oflimitation.

The foregoing description of specific embodiments reveals the generalnature of the inventive subject matter sufficiently so that others can,by applying current knowledge, readily modify and/or adapt it forvarious applications without departing from the general concept.Therefore, such adaptations and modifications are within the meaning andrange of equivalents of the disclosed embodiments. The inventive subjectmatter embraces all such alternatives, modifications, equivalents, andvariations as fall within the spirit and broad scope of the appendedclaims.

What is claimed is:
 1. A device comprising: a pressure sensor, saidpressure sensor including: a sense cell having a first electrode formedon a substrate and a sense diaphragm overlying and spaced apart fromsaid first electrode to produce a sense cavity, said sense cell beingconfigured to produce a first sense signal in response to pressure; atest cell having a second electrode formed on said substrate and a testdiaphragm overlying and spaced apart from said second electrode toproduce a test cavity, said test cell being configured to produce asecond sense signal in response to said pressure, wherein a firstsensitivity of said sense cell to said pressure is less than a secondsensitivity of said test cell to said pressure and a value of saidsecond sensitivity is utilized to estimate a value of said firstsensitivity; and a reference cell, said reference cell including a thirdelectrode formed on said substrate, a reference diaphragm overlying andspaced apart from said third electrode to produce a reference cavity,and a cap layer over said reference diaphragm, said cap layer makingsaid reference cell insensitive to said pressure.
 2. The device in claim1 wherein said pressure sensor further comprises a common electrodeoverlying and spaced apart from said first and second electrodes, saidcommon electrode including said sense diaphragm and said test diaphragm.3. The device in claim 1 wherein said pressure sensor further comprises:a set of sense cells formed on said substrate, said set of sense cellsincluding said sense cell; and a set of test cells formed on saidsubstrate, said set of test cells including said test cell, wherein saidset of test cells is configured in an interleaved arrangement with saidset of sense cells.
 4. The device in claim 1 wherein: said sensediaphragm is characterized by a first width; and said test diaphragm ischaracterized by a second width, said second width being greater thansaid first width.
 5. The device in claim 1 wherein said test cell isutilized to estimate said first sensitivity of said sense cell to saidpressure without application of a pressure stimulus that is greater thanatmospheric pressure.